Note: This may not work on Easy. In the Gearhead Vault, which you can access through Subway Town during (and after) the Gaining Influence Job, there are 7 computer terminals in an office area that can be activated. This is down the stairs from the bank vault area -- you'll want to go here after killing the final two heavy Gearheads. There are more than 7 computers, but only 7 of their keyboards make a DOOM sound when activated (you won't see an activation prompt). After you hit all 7 (some are off in separate offices), head back to the very beginning of the level. Jump on the flaming barrel and then onto the boxes next to the exit to Subway Town and you should be able to see a prompt to open the door here. Behind it is the warp to the DOOM room where you can grab the Space Marine Bobblehead.

In the Ghost Hideout, there's a sequence in the very first mission, Quell the Bandit Threat, where you are stabbed in a cutscene and taught how to use the Defibrillator. After this you exit the 'killing room' into a hotel room-like area, clear out all the bandits and head to the back of the room toward a painting on the wall. Between the TV and the painting in this room, you can open up a section of wall. Behind it is a pixelated Wolfenstein room with the Wolf Goblet inside. You can sell it for 150 dollars.

Note: This may not work on Easy. The Quake room, found in the Wasted Garage, can be accessed only after you press three well-hidden red buttons in the area. If you recall, the Wasted Garage is where you were asked to retrieve a distributor cap item during an earlier mission. It is recommended that you complete said mission and re-enter the Wasted Garage from the back entrance.

The three buttons are hard to find: one is located under a night stand, the other between a fallen file cabinet and the window, and the last behind a red tool cabinet. Each makes a Quake-like sound and glows for an instant when pressed.

After all three buttons have been activated, a hallway marked by a red Quake symbol leads to some stairs and to a portal that takes you to the hidden Quake room. Inside, you'll find the Shambler Plush in the center. To leave, you must step on the tile in the area from which you came.

Upon returning to the Ghost Hideout for the 'Missing Person' Job Board Quest, walk back over to where the zipline deposited you at the end of the first mission. Look to your right, on the blocked staircase, to find a box of 'Quayola Quayons', with the Quake logo as the Q's in the title. The box makes a Quake sound effect when picked up.

Just northwest of Mutant Bash TV, up a hill, you'll find Crazy Joe's place by a swamp (with Night Blossoms in it). A hermit by the name of Crazy Joe living here sports a Doom 3 shirt, as one of the many homages to id's other acclaimed franchises.

There must be some fans of AMC's Breaking Bad in the house. When you go to the sheriff's office in Wellspring, after talking to him turn and walk back into the holding cell area and on a shelf opposite the first cell you can find the cube that was presented to Hank Schrader with Tuco's "Grill" in it.

EditIn the Dead City in a small corridor at the bottom of the stairs there is a posterized image of a monster on the wall from Doom 3. The monster is only shown during the loading screen while in Hell in Doom 3 and is not actually faced during gameplay. Some have refered to the monster as the Icon of Sin but this is not officially confirmed.

In the mission "hijacked well", after you take the ghost toxin take the lift. After you take the lift there should be a room by the stairs. Go into the room and you will find a box of "Pinkies". It has pink beast on it and growls when you pick it up.